Plasma processing method and apparatus

ABSTRACT

A plasma process and an apparatus therefor are described. A number of substrates are disposed between a pair of electrodes, to which a high frequency electric power is applied in order to generate glow discharge and induce a plasma. The substrates in the plasma are applied with an alternating electric field. By virtue of the alternating electric field, the substrates are subjected to sputtering action.

This is a divisional application of Ser. No. 07/303,240, filed Feb. 3, 1989.

BACKGROUND OF THE INVENTION

The present invention relates to plasma processing method and apparatus.

Recently, a process for forming carbon films having a very high hardness has been developped utilizing chemical vapor reactions. An example is described in Japanese Patent Application No. SHO56-146936 filed on Sept. 17, 1981. Carbon films of this type are very useful because of their high hardness providing a smooth and corrosion-proof surface.

However, by use of the prior art apparatus and method, a large area or a number of substrates can not be treated at once. Particularly, when an uneven surface is desired to be coated or processed, only a few deposition takes place in depression surface portions.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a plasma processing method and apparatus capable of performing plasma process over a large processing area.

It is another object of the present invention to provide a plasma processing method and apparatus capable of producing much throughput of electric devices.

In order to accomplish the above and other objects and advantages, the plasma of reactive gas is created between a pair of electrodes to which a high frequency power is supplied, and a number of substrates are disposed within the plasma space and supplied with an alternating electric power. Grow discharge takes place by applying a high frequency power between the pair of electrodes. By virtue of the alternating power supplied to the substrates, sputtering action is effected thereto. The sputtering action functions, during deposition, to enable to deposit an amorphous film, and during etching, to increase the etching speed. The carbon film deposited in accordance with the present invention is, for example, a so-called diamond like carbon film whose Vickers hardness is expected to 6500 Kgt/mm² or higher depending on the formation condition. The energy gap is not lower than 1.0 eV, prefereably 1.5 to 5.5 eV. When used for thermal heads which are frequently subjected to rubbing action, the smooth and hard surface of the carbon film is very advantageous. On the other hand, when used for integrated circuits of semiconductor devices, the low thermal conductive coefficient of the carbon film makes it possible to dissipate heat generated in the integrated circuit and the circuit from suffering from a high temperature. It is also possible to fabricate a carbon film containing diamond particles by adjusting the deposition condition.

Phosphorous or diborane may be introduced together with a carbon containing reactive gas in order to make the deposited film be semiconductive. After completion of the deposition, the reaction chamber may be cleaned by effecting plasma etching with argon by the plasma process in accordance with the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a plasma chemical vapor reaction apparatus in accordance with the present invention.

FIGS. 2(A), 2(B) and 2(C) are schematic diagrams showing modifications of the substrate holder of the apparatus illustrated in FIG. 1.

FIGS. 3(A) to 3(C) are cross sectional views showing a fabrication method of carbon patterns in accordance with the present invention.

FIG. 4 is a schematic diagram showing another exemplary plasma processing apparatus in accordance with the present invention.

FIGS. 5(A) and 5(B) are plan views showing modifications of the substrate holder of the apparatus illustrated in FIG. 4.

FIGS. 6(A) to 6(D) are cross sectional views showing the deposition of carbon films on OPC printing drums.

FIG. 7 is a schematic diagram showing a modification of the apparatus illustrated in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 showing a plasma CVD apparatus, a carbon film deposition is explained in accordance with the present invention. The apparatus comprises a loading and unloading chamber 7', a reaction chamber 7 connected to the loading chamber 7' through a gate valve 9, a vacuum system provided for evacuating the reaction chamber 7 and consisting of a pressure control valve 21, a turbo molecular pump 22 and a rotary pump 23, a gas feeding system 10 for introducing a process gas into the reaction chamber 7 through nozzles 25, a substrate holder 1' functioning also as an electrode, a pair of mesh electrodes 3 and 3', and a high frequency electric energy supply 40 connected with the substrate holder 1 and the mesh electrodes 3 and 3' and consisting of a high frequency power source 15, a matching transformer 16, an AC power source 17 and a bias applying device 29. A temperature controlling means is provided in the chamber 7 in order to maintain the temperature of the reactive gas, e.g. between 450° C. and -100° C., although not illustrated in the figure. The output frequency of the high frequency power source 15 is 1 to 50 MHz, e.g. 13.56 MHz. The pair of electrodes 3 and 3' are connected respectively to the ends 4 and 4' of the secondary coil of the transformer 16. The substrate holder 2 is connected to the midpoint of the secondary coil through the AC power source 17 and the bias applying device 29. The frequency of the AC source is 1 to 500 KHz, e.g. 50 KHz. The plasma generating space between the electrodes 3 and 3' are confined by a cylindrical enclosure 2 whose upper and lower openings are closed by a pair of lids 8 and 8'. The substrate is a four-sided cylinder of 20 cm to 1 m height and 30 cm to 3 m width. Optionally, a microwave excitation device 26 may be provided in association with the gas feeding system 10. For example, the device 25 energizes the reactive gas supplied from the gas feeding system 10 by microwaves at 2.45 GHz and at 200 W to 2 KW. By use of this device 25, the deposition speed and the etching speed are increased about 5 times and about 4 times respectively.

A method of depositing carbon films will be described hereinbelow. Substrates to be coated are placed in the loading chamber 7'. After evacuating the loading and reaction chamber 7 and 7', the substrates 1 are transported to the reaction chamber 4 through the gate valve 9. In the illustration, the substrates 1 are dishes. An alminum film is coated on the rear surfaces of the dishes by vacuum evaporation. The substrate holder is adapted to support the dishes in electrical connection through the aluminum film.

A reactive gas is admitted to the reaction chamber 7 through the gas feeding system 10. The reactive gas includes a carbon compound gas such as CH₄, C₂ H₄, C₂ H₂, CH₃ OH or C₂ H₅ OH. For example, H₂ and CH₄ are inputted through the line 11 and the line 12 respectively at a same rate. The ratio of hydrogen contained in the carbon film to be subsequently deposited can be controlled by adjusting the proportion of hydrogen in the reactive gas. Preferably, hydrogen is contained in the films at not higher than 25 mol %. In addition to the reactive gas, a dopant gas may be introduced. Examples of impurity are N (not higher than 5 atom %), boron or phosphorus (0.1 to 5 atom %), a halogen (not higher than 25 atom %). The introduction is made respectively in the form of NH₃, B₂ H₆, PH₃, and C₂ F₆ or C₃ F₈.

A high frequency electric power of 13.56 MHz is applied to the reactive gas from the high frequency power source 15 at 0.5 to 5 KW. The output power is equivalent to a plasma energy of 0.03 to 3 W/cm². For example, the output power is 1 KW (0.6 W/cm²). On the other hand, an alternating electric power (voltage) of 50 KHz is applied to the substrate holder (the substrates 1) by the AC power source 17. The power applied to the substrates 1 is biassed by -200 V to +600 V by the bias device 39. The actual bias potential at the substrates 1 is -400 to +400 V since a self bias voltage is spontaneously applied thereto even without the application of an external bias. Preferably, the bias is applied so that the substrates functions as cathodes. As a result, a plasma gas is generated in the reaction chamber 7 and carbon deposition takes place on the substrates 1. The deposition speed reachs 100 to 1000 Å/min in a highly dense plasma. When the deposition is carried out without the microwave power and a 100 to 300 V bias voltage is superimposed on the electric power, the deposition speed is 100 to 200 Å/min. When the deposition is carried out with the microwave power and a 100 to 300 V bias voltage is superimposed on the electric power, the deposition speed is 500 to 1000 Å/min. The pressure of the reactive gas is maintained at 1 to 0.01 Torr, e.g. 0.1 Torr in the reaction chamber 4. The substrate temperature is 50° to 150° C. without particular heating. As a result, there is deposited an amorphous carbon film of 50 Å to 10 microns thickness on the substrates 1. The carbon film consists of an amorphous carbon which may contain, depending on the deposition condition, microcrystals of 5 Å to 2 microns diameter. The Vickers hardness of the carbon film is not lower than 2000 Kg/mm² and the thermal conductivity is not lower than 2.5 W/cm deg, preferably 4.0 to 6.0 W/cm deg. The carbon film is characterized by C--C bonds of sp³. Exhausted gas is removed through the evacuating system 21 to 23.

An electric power at 1 GHz or higher frequency, e.g. 2.45 GHz can break C--H bonds while C--C, C═C can be broken at 0.1 to 50 MHz, e.g. 13.56 MHz. The amount of hydrogen involved in the carbon films is preferably not higher than 25 mol %. Optionally, it is usuful to add impurity such as phosphorus, nitride or boron in order to fabricate carbon n-type or p-type semiconductors.

The substrate holder may be modified in accordance with the application. FIGS. 2(A) and 2(B) are schematic diagrams showing examples. In FIG. 2(A), a number of substrate holders 1' in plate from are integrally formed in the enclosure 2. A pair of substrates 1 are mounted on opposed sides of each holder 1'. The distance between each adjacent holders is chosen to be 6 to 10 cm. The size of the holder is 60 cm×30 cm. The substrates are mounted on the holders, saving the marginal portions of 10 cm from the lateral ends and 5 cm from the upper and lower ends respectively. Then, the thickness of the carbon film becomes 1 micron ±5%. In order to suspend pincettes 1--1, a matalic mesh 2' may be provided in the enclosure 2 as shown in FIG. 2(B). Also, vessels 1-2 may be placed on a mesh 2".

Although the rear surface of the substrate to be coated is preferably made conductive, the provision of a conductive coating may be dispensed with. An example is described in conjunction with FIG. 2(C). This example is devised to treat a number of drinking glasses. In the enclosure 2, a number of supporting members 2'-1 to 2'-n made of a conductive material such as alminum. The members are provided with glass receivers made of stainless steel in the form of glasses and adapted to make contact with the inner surface of glasses to be supported. In accordance with experiments, when the clearance between the inner surface of the glasses and the receivers is as narrow as 2 mm, carbon deposition could be carried out in the same manner as with the aluminum film directly coated on the inner surface of the glasses. However, when the distance between the glasses and the receivers reached about 10 mm, the effect of the conductive receivers could not be confirmed. Alternatively, alminum foils may be interposed between the receivers and the glasses or replace the receivers.

The present invention can be applied to etching process. After forming carbon films on a substrate, a mask is coated over the film and placed in the chamber, followed by introducing an etchant gas into the reaction chamber 7. The etchant gas is for example O₂, air, NO₂, NO, N₂ O, a mixture of oxygen and hydrogen, or another suitable oxygen compound. The plasma gas is generated in the same manner as in the deposition process to perform etching of the carbon film. Preferably, the substrate temperature is maintained at 100° to 100° C. during etching. The pressure of the reactive gas is 0.01 to 1 Torr. An experiment is explained below.

A semiconductor device using a carbon film pattern was produced. The process is explained in conjunction with FIGS. 3(A) to 3(C). The device was formed within and on a silicon semiconductor substrate 31. The substrate, a silicon oxide film 37 and a superconducting ceramic film 32 were formed and patterned respectively, followed by coating of a 0.5 micron blocking film 33 which prevents the underlying film 32 from being exposed to etching action. The blocking film 33 was made of SiO₂. Then, a carbon film 34 was deposited on the structure to a thickness of 0.1 to 2 microns, e.g. 0.5 micron. A photoresist pattern was formed on the carbon film 34 with an opening 36. The opening 36 is a bonding pad for making contact with an IC chip. The thickness of the photoresist pattern was selected so as not to expose the underlying surface, even if it was decreased by the following etching.

Next, oxygen gas was introduced to the reaction chamber and a high frequency power was inputted at 300 W to perform etching of the carbon film 34. After completion of the etching, the photoresist 35 and a portion of the blocking film 34 at the opening was removed by etching using NF₃ or SF₆.

Alternatively, the conductive film 32 may be formed of Al, Ag or Si. The blocking film 33 may be formed of a phosphosilicate glass. The thickness of the blocking film may be 0.1 to 2 microns in general. Instead of the thick photoresist pattern, a silicon oxide pattern may be formed by coating silicon oxide film and etching the same with a photoresist. Silicon oxide does not suffer etching action from oxygen.

Superconducting ceraics for use in accordance with the present invention may be prepared in consistence with the stoichiometric formulae (A_(1-x) B_(x))_(y) Cu_(z) O_(w), where A is one or more elements of Group IIIa of the Periodic Table, e.g. the rare earth elements, B is one or more elements of Group IIa of the Periodic Table, e.g. alkaline earth elements, and x=0.1-1; y=2.0-4.0, preferably 2.5-3.5; z=1.0-4.0, preferably 1.5-3.5; w=4.0-10.0, preferably 6.0-8.0. Also, superconducting ceramics for use in accordance with the present invention may be prepared consistent with the the stoichiometric formulae (A_(1-x) B_(x))_(y) Cu_(z) O_(w), where A is one or more elements of Group Vb of the Periodic Table such as Bi, Sb, and As; B is one or more elements of Group IIa of the Periodic Table, e.g. alkaline earth elements, and x=0.3-1; y=2.0-4.0, preferably 2.5-3.5; z=1.0-4.0, preferably 1.5-3.5; w=4.0-10.0, preferably 6.0-8.0. Examples of the latter general formulae are Bi₄ Sr_(y) Ca₃ Cu₄ O_(x), Bi₄ Sr_(y) Ca₃ Cu₄ O_(x), Bi₄ Sr_(y) Ca₃ Cu₄ O_(x), Bi₄ Sr_(y) Ca₃ Cu₄ O_(x), Bi₄ Sr_(y) Ca₃ Cu₄ O_(x), Bi₄ Sr_(y) Ca₃ Cu₄ O_(x) (y is around 1.5).

Referring now to FIG. 4, another embodiment is described. The similar elements as those described in FIG. 1 are given similar numerals and no redundant explanation for the elements will be repeated.

Particularly, the power source 41 features the present embodiment. The source 41 comprises a pair of high frequency sources 45-1 and 45-2 which are coupled by a phase shifter 46. The differential phase between the outputs of the sources 45-1 and 45-2 is controlled by means of the phase shifter 46. The outputs of the sources 45-1 and 45-2 are coupled with a pair of matching transformers 46-1 and 46-2 which, in turn, are coupled in series at respective one terminals of the secondary coils of the transformers. The other terminals of the transformers 46-1 and 46-2 are connected to the pair of meshed electrodes in the same manner as the first embodiment. An AC power source 47 and a bias device 29 are connected between the pair of meshed electrodes 3 and 3' in the same manner as the first embodiment.

The relative phases of the electric powers applied to the electrodes 3 and 3' are controlled by means of the phase shifter 46. Namely, the potentials at the pair of the electrodes 3 and 3' are maintained exactly in phase or opposite phases. The phase difference may be adjusted at any suitable angle other than 0° or 180° in accordance with the condition of the electrodes in the circumference.

Next, an exemplary deposition method by use of the apparatus is explained. A number of printing drums 1 for electrostatic copying machines are mounted on substrate holders which are integrally formed in an enclosure 42. The surface of the printing drums is coated with an organic photoconductive film in advance. The mounting condition of the drums in the rectangular enclosure of 75 cm width×75 cm depth×50 cm height is illustrated in FIG. 5(A). In the figure, 16 drums are mounted while dummy drums 1-0 are provided along the inner surface of the enclosure in order to make uniform the electric field induced in the plasma generating space 60. FIG. 5(B) shows drums mounted in a hexaglnal enclosure.

NF₃ and C₂ H₂ are introduced into the reaction chamber as a reactive gas. The ratio of NF₃ /C₂ H₂ is 1/4 to 4/1, e.g. 1/1. By virtue of the high frequency power from the power source 45, plasma is induced and a carbon film is depostied on the drums. The carbon film is formed while being subjected to sputtering action which is mainly induced by the power from the power source 47. As a result, the carbon film becomes a diamond like carbon film containing nitrogen at 0.3 to 10 atom % and fluorine at 0.3 to 3 atom %. The transparency and the resistivity of the diamond like carbon can be controlled by varying the proportions of the nitrogen and fluorine contain therein. From the view point of the application to printing drums, a suitable resistivity may be found in the range of 1×10⁷ to 1×10¹⁴ ohm cm, preferably 1×10⁹ to 1×10¹¹ ohm cm.

FIG. 6(A) is a cross sectional view showing the printing drum. The drum comprises an alminium cylinder 51 whose one end is closed and provided with a projection for enabling the rotatably supporting of the drum in a copying machine. The inner surface of the other end of the drum is threaded at 53. A photosensitive dual film 57 consisting of an organic photoconductor film and a carrier transmission film is coated on the alminium cylinder 51. The carbon film 54 is formed covering the dual film 57. The thickness of the carbon film is 0.1 to 3.0 microns. FIG. 6(B) is an expanded partial view of FIG. 6(A). Since the drum is repeatedly subjected to rubbing action, there may occur the rubbing off of the end portions 11 of the carbon film. In order to strengthen the end portions, it is desirable to thicken the end portions as illustrated in FIG. 6(C). The thickened portion 55 can be formed by locating the end portions near the meshed electrodes 3 and 3' and adjusting the plasma pressure in a range of 0.08 to 0.1 Torr. The configuration in FIG. 6(B) is realized when the plasma pressure is 0.05 Torr. Otherwise, the end portions of the carbon film may be dispensed with as shown in FIG. 6(D). This can be realized by adjusting the plasma pressure in a range of 0.01 to 0.04 Torr or by hiding the end portions with suitable covers.

After the completion of the deposition, the drums are removed from the chamber, and the inside of the chamber in which unnecessary carbon may be deposited is cleaned by introducing an etchant of oxygen and effecting plasma etching. Subsequently, H₂ plasma etching is effected to eliminate oxide contaminants attached on the inside of reaction chamber. Other deposition and etching conditions are same as the plasma process of the first embodiment.

A modification of the apparatus shown in FIG. 4 is illustrated in FIG. 7. Similar numerals are given to the similar elements in FIG. 7 as in FIG. 4. 56-1 and 56-2 are matching boxes of LCR with which the impedance matching between the electrodes 3 and 3' and the high frequency electric power source 45-1 and 45-2 is set up. The electric power sources are provided with earth lines 55-1 and 55-2. The frequency of one of the electric power sources is equal to or a multiple of that of the other. An intermediate voltage supply is connected to the substrate holder 2 and comprised of an AC voltage source 57-1, a DC voltage source 57-2, a three-way switch 51. The DC voltage source 57-2 functions to superimpose a DC bias voltage on the AC voltage from the AC volatage source 57-1. The frequency of the AC voltage source is 10 Hz to 100 KHz. With the switch 51, three states of bias application can be selected. The substrates 1 are rendered electrically floating at 51-1, applied with the AC voltage from the AC source 57-1 biassed by the DC voltage of the source 57-2 at 51-1, and earthed at 51-3. In this case, the input energy is 0.5 to 50 KW (0.005 to 5 W/cm²), for example, 1 KW (0.1 W/cm²).

The phase difference between the voltages at the electrodes 3 and 3' is adjusted in order to spread plasma gas over the whole deposition space. If the deposition system is exactly symmetrical with respect to the electrodes 3 and 3', the phase difference is 0° or 180°. However, depending on the asymmetry, the optimum value is displaced from 0° or 180°. In accordance with experiments, the optimum values were in a range of ±30° or 180° ±30°. Plasma gas was collected mainly near one of the electrodes when the phase difference was around 90° or 270° (±30°).

Carbon films were deposited on printing drums 1 coated with organic photoconductive films in accordance with the modification. The reactive gas consisting of C₂ F₆ /C₂ H₄ (=1/4 to 4/1) was introduced to the chamber downwardly from nozzles 65. The drums were not particularly heated and maintained in a range of room temperature to 150° C. A negative bias voltage of -50 to -600 V was established by applying an AC bias voltage from the intermediate voltage supply. Other deposition conditions were same as the first embodiment. Then, diamond like carbon films containing fluorine of 0.1 to 8 microns thickness were formed on the drums at a deposition speed of 1000 Å/min.

The formation was carried out in two steps. First, ethylene and hydrogen were used in order to deposit carbon thin films of 0.01 to 0.1 micron which had a high adhesivity to the organic surface of the drums. The resistivity of the films was 1×10⁶ to 5×10¹³ ohm cm. Next, 0.2 to 2 microns thick carbon films were deposited on the underlying thin carbon films by use of a reactive gas consisting of C₂ F₆ and NH₃. The resistivity of the films were 1×10⁷ to 5×10¹² ohm cm.

Referring to FIG. 8 (A), the deposition of silicon nitride films and fluoride containing carbon films is explained. Glass substrates 61 and 61' are supported on the substrate holder 2. The substrates are, for example, front, side and rear window panes and side mirrors of motor-cars, motor-cycles and airplanes, or windows of buildings. In this embodiment, a silicon nitride film 65-1 is formed on the substrate in advance of carbon coating 65-2 because glass tends to react with fluoride. The resistivity of the carbon films is adjusted to be 1×10⁶ to 5×10¹³ ohm cm by controlling the introduction of fluoride in order to prevent from gathering dust due to electrostatic force. FIG. 8(B) shows the deposition on a curved surface.

While several embodiments have been specifically described, it is to be appreciated that the present invention is not limited to the particular examples described and that modifications and variations can be made without departure from the scope of the invention as defined by the append claims. Examples are as follow.

Since the thermal conductive coefficient of carbon film formed in accordance with the present invention is very high, thermal diffusion of IC chips may be expedited by coating the rear surfaces with the carbon film in advance.

The substrate to be coated in accordance with the present invention may be made of any material, such as plastics, e.g. PET (polyethylenetelephtate), PES, PMMA, tefon, epoxy, polyimide or other organic resin. The shape of the substrate may includes several types, e.g. such having irregular surfaces.

The materials to be deposited on a substrate in accordance with the present invention include, other than carbon, silicon oxside, silicon nitride, or other materials.

An excellent speaker can be realized by utilizing a vibration cone whose inside and outside surfaces have been coated with a very hard carbon film in accordance with the present invention. The both side surface can be coated at once in a reaction chamber. 

We claim:
 1. A plasma processing method comprising:disposing in a reaction space a plurality of printing drums where each drum has a photosensitive coating disposed thereon; supplying a reactive gas comprising carbon into said reaction space wherein said reaction space is defined between a pair of electrodes where a first high frequency power source is connected to the first of said pair of electrodes and a second high frequency power source is connected to the second of said pair of electrodes and where the first high frequency power source is phase shifted with respect to the second high frequency power source; inducing a plasma of the reactive gas in said reaction space by actuating said first and second high frequency power sources; and forming a carbon containing coating on each of the photosensitive coatings of the printing drums.
 2. A method as in claim 1 where each of said photosensitive coatings comprises an organic photoconductor.
 3. A method as in claim 2 where said carbon containing coating is a diamond like carbon coating.
 4. A method as in claim 3 where the thickness of the diamond like carbon coating is 0.1 to 3.0 microns.
 5. The method of claim 1 wherein the differential phase between the first and second high frequency power sources is in a range of -30° to ±30° or 150° to 210°.
 6. The method as in claim 1 including applying an alternating electric power, whose frequency is lower than that of said first and second high frequency power sources, to said printing drums during the plasma state reaction in order to facilitate sputtering action on said drums.
 7. The method of claim 6 wherein the frequency of at least one of said first and second high frequency power sources is in the range of 1 to 50 MHz.
 8. The method of claim 7 wherein said alternating power is selected in a range of 1 to 500 KHz.
 9. The method of claim 1 wherein said printing drums are negatively biased.
 10. The method of claim 1 wherein said electrodes are mesh electrodes.
 11. A plasma processing apparatus comprising:a reaction chamber; a gas feeding system for introducing a reactive gas comprising carbon into said reaction chamber; a vacuum pump for evacuating said reaction chamber and maintaining a predetermined gas pressure in said reaction chamber; a pair of electrodes between which a plasma generating space is defined; at least one substrate holder for supporting in said plasma generating space a plurality of drums where each drum has a photosensitive coating disposed thereon so that a carbon containing coating is formed on each photosensitive coating; first and second high frequency power sources respectively connected to said pair of electrodes; and phase shifting means for shifting the phase of the first high frequency power source with respect to the second high frequency power source.
 12. An apparatus as in claim 11 where each of said photosensitive coatings comprises an organic photoconductor.
 13. An apparatus as in claim 12 where said carbon containing coating is a diamond like carbon coating.
 14. An apparatus as in claim 13 where the thickness of the diamond like carbon coating is 0.1 to 3.0 microns.
 15. An apparatus as in claim 12 wherein the differential phase between the first and second high frequency power sources is in a range of -30° to +30° or 150° to 210°.
 16. An apparatus as in claim 11 including an alternating power supply for applying an alternating voltage to said printing drums, the frequency of the alternating voltage being chosen so that the printing drums are substantially subjected to sputtering action of the plasma generated in the plasma generating space. 